Focused ultrasound (i.e., acoustic waves having a frequency greater than about 20 kilohertz) can be used to image or therapeutically treat internal body tissues within a patient. For example, ultrasonic waves may be used to ablate tumors, eliminating the need for the patient to undergo invasive surgery. For this purpose, a piezo-ceramic transducer is placed externally to the patient, but in close proximity to the tissue to be ablated (“the target”). The transducer converts an electronic drive signal into mechanical vibrations, resulting in the emission of acoustic waves. The transducer may be shaped so that the waves converge in a focal zone. Alternatively or additionally, the transducer may be formed of a plurality of individually driven transducer elements whose phases can each be controlled independently from one another and, thus, can be set so as to result in constructive interference of the individual acoustic waves in the focal zone. Such a “phased-array” transducer facilitates steering the focal zone to different locations by adjusting the relative phases between the transducers. Magnetic resonance imaging (MRI) may be utilized to visualize the focus and target in order to guide the ultrasound beam.
To minimize, for a given focus intensity, the intensity experienced by tissue regions surrounding the target, the ultrasound waves preferably come from multiple directions. This may require large transducer surfaces that surround the target as much as possible. Large-area transducers arrays may, however, be difficult to handle, and may complicate beam steering. Moreover, the placement of the transducer may be constrained by anatomical barriers, depending on the particular target location inside the body. For the treatment of different tissues and organs, differently shaped transducers may be desirable. Further, even if the location of the transducer with respect to the target is geometrically optimal, ultrasound waves coming from certain regions of the transducer surface may be blocked by strongly absorbing tissue, such as, e.g., bone, or other internal anatomical barriers. Accordingly, there is a need for focused ultrasound transducer arrays that are adjustable to various anatomical constraints.